Can AMP be used as both mobile and desktop versions?
I didn't find a straight answer on that anywhere as it clearly say in the name (AMP) itself too, that it's used for a static mobile web pages.
I found out that amp tag is used on desktop version like for example ampproject.org has
So it's unclear to me now whether the AMP can (is benefitial to) be used on desktop version as well?
Answer
It is possible, your site is going to work well on both Desktop and Mobile devices, it's going to load properly and work really fast.
BUT, keep in mind that AMP is more oriented toward mobile devices, all of the current libraries that you can use are definitely focused in creating a faster experience on small devices, and some of the mayor sites implementing AMP are using different version, like Mobile, Desktop and AMP version of each page or article, sounds complicated to maintain, but it allows to offer the best experience for each user on every device.
So, if you want to create a cross device site from start, AMP is a good option, just keep in mind that a bunch of the things you see on other sites (interaction plugins, javascript libraries, complex ajax functionality, complex forms and validations) are not currently supported, maybe in the future, maybe not.